How Do You Measure the Size of the Universe?

The observable universe is about 90 billion light-years across today—roughly a sphere with a radius near 46 billion light-years—and astronomers can estimate that size without ever “measuring” the cosmos with a ruler. The key is turning the universe’s age into a distance, then correcting for the fact that space itself has been expanding while light travels.

Start with the universe’s age, currently estimated at about 13.8 billion years. Light emitted at the earliest times has had at most that long to reach us, so a naive guess might place the edge of what we can see at 13.8 billion light-years away. The actual radius is much larger because the distance light covers is not just a matter of time and speed: the space between galaxies stretches. In the raisin-bread analogy, galaxies (the raisins) don’t expand, but the dough (space) rises, pushing raisins farther apart. Over cosmic history, that stretching can happen at different rates—fast in some eras, slower in others—so the “distance traveled” by a photon depends on the entire expansion timeline, not just the present-day expansion rate.

To reconstruct that expansion history, astronomers rely on cosmological redshift, a wavelength shift caused by expanding space. Light’s color is tied to its wavelength; if space were static, light would arrive with the same wavelength it left with. Instead, expansion stretches wavelengths, shifting light toward the red end of the spectrum. The farther away a galaxy is, the longer its light has traveled through expanding space, and the more its wavelength has stretched—so redshift becomes a measurable proxy for distance and time.

The crucial quantitative step is mapping the distance–redshift relationship. By observing many galaxies with measured redshifts and independently estimated distances, researchers fit a curve that links redshift to distance. That relationship encodes how quickly space expanded at different moments in the universe’s past.

With the expansion history in hand, the calculation of the observable-universe size follows a second logic chain. Run the cosmic “movie” backward using the inferred expansion rates until the universe collapses to the Big Bang—this yields the age again, about 13.8 billion years (give or take). Then run the movie forward: imagine light emitted immediately after the Big Bang from every point in space. The particular beam that arrives exactly when the universe is 13.8 billion years old marks the edge of the observable universe. That boundary lands at roughly 46 billion light-years in radius, or about 90 billion light-years in diameter.

Beyond that lies the unobservable universe—regions whose light has not yet reached us. Those farther domains exist in principle, but pinning down their size requires additional assumptions and is left for another discussion.